when to run up and check the prop

Ok, brother flyers, cards on the table here. I always do a run-up
before taking the active (my Mooney only sees pavement, so gravel
damage isn't a serious consideration) as do I think most of you. I'm a
M20J jock. I have, in some 3000 hours, returned to base after taxing
out only about 4 times. Once, in a normally inducted Mooney Ranger the
carb heat wire broke during that test (quite a surprise when you pull
out a foot of wire!), a couple of time because a radio wasn't working,
once because a bank of spark plugs weren't firing. Never had to come
back because the prop didn't cycle correctly.

I am NOT saying don't cycle the prop! I am wondering what failure modes
you guys have experienced during run up.

I've had in flight mag bank failures, vacuum pump failures, alternator,
radio, and common sense failures too. All but the last lead to a
landing at the nearest suitable airport (ever notice how often a
failure is in solid IFR?). The common sense failure modes did not
result in landing at the nearest airport or turning around, but they
should have.

OK, I've shown you mine. Let's see yours.